Weather Whys: Why bridges ice before roadways

Weather Whys

With all this talk about freezing rain and sleet, we should address a question many of you have about a certain sign you see on the roadways during the cold weather months. 

You see those yellow signs before you cross a bridge that say, “watch for ice on bridge” or “bridge ices before road” But do you know why this happens?

Those of you who drive for a living may know this answer but you’d be surprised that it’s not widely known. The answer is actually very simple. 

A bridge is suspended away from the ground and away from the heat of the Earth. All of the cold air surrounds the bridge and allows it to cool down faster and creates ice faster than the road attached on the earth.This is why road crews will often treat bridges heavily during winter storms.

It isn’t just bridges that we have to worry about. It’s any road that is suspended like exit and on ramps to highways. You even have to watch out for underpasses because bridges can drop ice or water that will freeze to the roadways below. If there is a light flashing with the “bridges ice before road” sign, heed that warning because ice could be forming or already has formed. Something else to consider about ice formation on the roadways is that snow and ice collect on concrete faster than asphalt because the asphalt can hold heat longer. 

But you should never use that guideline to decide how you’re going to drive. If the temperatures are below freezing, always assume there is ice and always drive with caution.

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